There are many types of signal generators that provide test signals for a variety of uses. Typically, a signal generator includes an amplitude control system that sets and maintains the amplitude, or level, of test signals provided by the signal generator at a test port. However, in some uses of the signal generator, signal conditioners, such as amplifiers, mixers, limiters, or filters are connected to the test port to further condition the test signal. Often, these signal conditioners are external to the signal generator. For example, when an external amplifier is connected to the test port, a conditioned signal is provided at the output of the amplifier that is an amplified version of the test signal provided by the signal generator at the test port. The amplitude control system is operated in either an internally-leveled configuration or an externally-leveled configuration in an attempt to set and maintain the amplitude of this conditioned signal at the output of the signal conditioner.
FIG. 1A shows an example of a prior art amplitude control system operating in the internally-leveled configuration. In this example, an external amplifier is coupled to the test port of the signal generator within which the amplitude control system is included. A signal separator 2 couples a portion of the signal 3 provided by a signal source to an internal detector before an input of the amplifier. The internal detector senses the coupled portion of the signal 3 and produces a detected signal 5 that corresponds to the amplitude of the signal 3 at the input of the amplifier. The detected signal 5 is received by an amplitude controller 4, such as an automatic level control circuit (ALC), which in response to the detected signal 5, drives the signal source to control the amplitude of the signal 3 that is applied to the amplifier. Thus, the amplitude of the conditioned signal 7 at the output of the amplifier is set by detecting and adjusting the amplitude of the signal 3 provided to the input of the amplifier.
Because the amplifier in this internally-leveled configuration is outside a feedback loop provided by the internal detector, the amplitude controller 4, and the signal source, gain drifts and non-linearities of the amplifier are not accommodated by the feedback loop. Thus, while the amplitude of the signal 3 present at the input to the amplifier is set and maintained in this internally-leveled configuration, the amplitude of the conditioned signal 7 provided at the output of the amplifier varies according to the gain drifts and other time-dependent instabilities of the amplifier. Non-linearities such as distortion, saturation or compression in the amplifier also result in amplitude changes at the output of the amplifier that do not correspond to changes in the amplitude of the signal 3, which makes it difficult to control the amplitude of the conditioned signal 7 at the output of the amplifier by adjusting the amplitude of the signal 3 provided at the input of the amplifier.
In addition, when the prior art amplitude control system is included in a signal generator for use with a network analyzer, the gain drifts and non-linearities of the amplifier cause amplitude tracking errors between the conditioned signal 7 applied to a device under test (DUT) and a reference signal 9 derived from the signal 3 via a signal coupler 6. Because accurate amplitude tracking of the reference signal 9 and conditioned signal 7 is relied upon to determine the amplitude of the conditioned signal 7 applied to the DUT, the amplitude tracking errors degrade the accuracy of the network analyzer with which the signal generator is used. Further, in this internally-leveled configuration, the impedance match ZOUT presented to the DUT is determined primarily by the output impedance of the amplifier and does not benefit from an improved impedance match that would be provided to the DUT were the amplifier included within the feedback loop.
FIG. 1B shows an example of the prior art amplitude control system operating in the externally-leveled configuration with the external amplifier, an external coupler and external detector coupled to the test port. The external coupler couples a portion of the conditioned signal 7 provided at the output of the amplifier to the external detector. The external detector senses the coupled portion of the conditioned signal 7 and produces a detected signal 11 that corresponds to the amplitude of the conditioned signal 7 at the output of the amplifier. The detected signal 11 from the external detector is provided to the amplitude controller 4 through a selection switch or other multiplexer (not shown). In response to the detected signal 11 from the external detector, the amplitude controller 4 drives the signal source to control the amplitude of the signal 3 to achieve a designated amplitude setting of the conditioned signal 7 provided to the DUT.
Because the amplifier in this externally-leveled configuration is within the feedback loop provided by the external detector, amplitude controller 4, and the signal source, gain drifts and non-linearities of the amplifier are accommodated by the feedback loop so that the effects of these amplifier characteristics on the amplitude of the conditioned signal 7 are suppressed by the feedback loop. However, when the prior art amplitude control system is included in a signal generator for use with a network analyzer, the amplitude tracking error between the conditioned signal 7 at the output of the amplifier and the reference signal 9 still results due to the inherent non-linearities of the amplifier. This amplitude tracking error that results in the externally-leveled configuration makes the prior art amplitude control system not well-suited for signal generators that are used with network analyzers. In addition, since the amplitude controller 4 is typically calibrated for transfer characteristics of the internal detector, offset and linearity errors result in the amplitude of the conditioned signal 7 when the amplitude of the conditioned signal 7 is set according to the detected signal 11 from the external detector. These errors in amplitude settings result from the external detector having a transfer function that is generally different from the transfer function of the internal detector for which the amplitude controller 4 is calibrated.
There is a need in signal generators to overcome the amplitude tracking errors, the difficulties in setting the amplitude of the conditioned signals 7, and other shortcomings that result when amplifiers and other signal conditioners are included in the prior art amplitude control systems shown in FIGS. 1A-1B.